X.A. Fan

Characterization and thermoelectric properties of p-type 25%Bi2Te3–75%Sb2Te3 prepared via mechanical alloying and plasma activated sintering

In the present work, starting from elemental bismuth, antimony and tellurium powders, p-type 25%Bi2Te3–75%Sb2Te3 thermoelectric materials with high density were prepared by mechanical alloying (MA) and plasma activated sintering (PAS). The single phase 25%Bi2Te3–75%Sb2Te3 alloys were obtained after MA for 12 h. The effect of sintering temperatures on microstructure and thermoelectric properties of the as-PASed samples was researched. Highly compact samples with relative density over 99% could be obtained when sintering temperature was over 653 K. A preferentially orientated microstructure with the (1 1 0) plane parallel to and the basal planes (0 0 l) perpendicular to the pressing direction was formed, and the orientation factors of the (0 0 l) planes changed from 0.11 to 0.12 at different sintering temperatures. The maximum power factor and figures of merit (Z) at room temperature were 3.10 × 10−3 W m−1 K−2 and 2.85 × 10−3 K−1, respectively. The Vickers microhardness reached 112.7 Hv, which was twice that of the single crystal samples prepared by zone-melting.

Bi2Te3 hexagonal nanoplates and thermoelectric properties of n-type Bi2Te3 nanocomposites

Bi2Te3 plate-like crystals with homogeneous hexagonal morphology were rapidly synthesized using a microwave assisted wet chemical method in 30 min. These Bi2Te3 nanoplates possessed a fixed edge with a length of ~0.5–2 µm, and the thickness was less than ~100 nm. The n-type Bi2Te3 nanocomposites were prepared by consolidating mixtures of these nanoplates and mechanically alloyed powders using plasma activated sintering, and the effect of nanoplate addition on the thermoelectric properties of the nanocomposites was investigated. When the content of the doped nanoplates was 15 wt%, the lattice thermal conductivity of the Bi2Te3 nanocomposites decreased by 18% compared with that of the undoped compounds. A preliminary investigation showed that nanopowder addition was an effective way to decrease the thermal conductivity and increase the thermoelectric efficiency.

Effect of nominal Sb2Te3 content on thermoelectric properties of p-type (Bi2Te3)x(Sb2Te3)1−x alloys by MA–HP

p-type (Bi2Te3)x(Sb2Te3)1−x alloys with homogeneous and fine microstructure were prepared by the mechanical alloying–hot pressing (MA–HP) method in the present work. X-ray diffraction, energy dispersive x-ray spectroscopy, scanning electron microscope–backscattered electron imaging were performed to characterize the MA–HPed materials. The effect of nominal Sb2Te3 content on mechanical and thermoelectric properties of the (Bi2Te3)x(Sb2Te3)1−x was investigated. Thermoelectric properties were measured at 300 K. By increasing the nominal molar fraction of Sb2Te3 from 0.7 to 0.9, the carrier concentration (nc) increased obviously from 0.43 × 1019 to 3.08 × 1019 cm−3; however, carrier mobility (μ) fluctuated between 198.0 and 285.5 cm2 V−1 s−1, and the Seebeck coefficient (α) and electrical resistivity (ρ) decreased acutely. The thermal conductivity (κ) increased with increase in the nominal Sb2Te3 content, while the lattice thermal conductivity (κph) decreased abruptly from 0.820 to 0.297 W m−1 K−1. When the nominal molar fraction of Sb2Te3 was 0.8, the resultant maximum power factor (PF) and thermoelectric figure of merit (Z) of the p-type (Bi2Te3)x(Sb2Te3)1−x alloys reached 3.81 × 10−3 W m−1 K−2 and 3.23 × 10−3 K−1 at 300 K, respectively. The bending strength reached 64 MPa, which was 3–5 times larger than that of single crystal materials.

Preferential orientation and thermoelectric properties of n-type Bi2Te2.85Se0.15 alloys by mechanical alloying and equal channel angular extrusion

Starting from elemental bismuth, tellurium and selenium powders, n-type Bi2Te2.85Se0.15 solid solution with a fine microstructure was prepared by mechanical alloying and equal channel angular extrusion (ECAE) in the present work. The effect of extrusion temperature on the microstructure and thermoelectric properties of the as-ECAEed samples was investigated. A preferentially oriented microstructure with the basal planes (0 0 l) in the parallel direction to extrusion was formed, and the orientation factors F of the (0 0 l) planes of the 703 K and 753 K ECAEed Bi2Te2.85Se0.15 alloys were 0.26 and 0.28, respectively. The electrical resistivity and the Seebeck coefficient decreased, and the thermal conductivity increased with increasing extrusion temperature. The electrical and thermal transmission performances were strongly affected by the preferentially oriented microstructure and the preferential orientation improved the thermoelectric properties of the ECAEed Bi2Te2.85Se0.15 alloys in the parallel direction to extrusion. The maximum dimensionless figure of merit was obtained when extruded at 753 K at a testing temperature of 343 K, ZT = 0.66.

Thickness and temperature dependence of electrical resistivity of p-type Bi0.5Sb1.5Te3 thin films prepared by flash evaporation method

P-type Bi0.5Sb1.5Te3 thin films with thicknesses in the range 80–320 nm have been deposited by the flash evaporation method on glass substrates at 473 K. XRD and field emission scanning electron microscope were performed to characterize the thin films. The results show that the thin films are polycrystalline and the grain size of the thin films increases with increasing thickness of the thin films. Compositional analysis of the thin films was also carried out by energy-dispersive x-ray analysis. A near linear relationship was observed between the electrical resistivity and the inverse thickness of the annealed thin films, and it agrees with Telliers model. Electrical resistivity of the annealed thin films was studied in the temperature range 300–350 K, and their thermal activation behaviour was characterized, the activation energy for conduction decreases with increasing thickness of the thin films.